1. Field of the Invention
The present invention relates to a control system for turbochargers for use with internal combustion engines of motor vehicles.
2. Description of the Prior Art
Nowadays, small-sized superchargers are installed even on popular passenger cars so as to improve the engine output torque during low and intermediate speed operations. However, because the engine would be damaged if the supercharging pressure rises to an unduly high level during a high speed engine operation, a part of the engine exhaust gases is caused to bypass the supercharger to keep the supercharging pressure within a predetermined level during high speed engine operations. For this purpose, the supercharger is provided with a controlling system including an exhaust gas bypass passage in which an exhaust gas bypass valve is provided. The supercharging pressure is utilized to control the operation of the bypass valve.
The pressure level of the supercharging pressure can be detected by electrical or mechanical system. The electrical system utilizes a pressure sensor operative to transduce detected pressures into an electrical signal. When the electrical signal rises beyond a predetermined level, an actuator is operated to drive the bypass valve to an open position. The mechanical system utilizes a diaphragm having opposite faces subjected to the supercharging pressure and to the atmospheric pressure, respectively. The diaphragm is operatively connected by a rod to the exhaust gas bypass valve. When the pressure differential between the supercharging and atmospheric pressures exceeds a predetermined value, the diaphragm is deformed to actuate the bypass valve to an open position. Compared with the electrical systems, the mechanical systems can be manufactured at a lower cost and provide more reliable operations. For this reason, the mechanical systems are in wide use on commercially available motor vehicles.
The mechanical systems, however, have a following problem: In order that engine exhaust gases may be released bypassing a turbocharger when the supercharging pressure exceeds, for example, 400 mmHg, the bypass valve may be opened when the pressure differential between the supercharging and atmospheric pressures respectively acting on the opposite faces of the diaphragm exceeds 400 mmHg. Assuming that a motor vehicle equipped with a turbocharger having a bypass valve actuated by such mechanical control system is operated in high altitudes, the atmospheric pressure is reduced by 9 mmHg as the altitude is increased by 100 meters. In other words, the atmospheric pressure is substantially 670 mmHg at 1,000 meters above sea level; therefore, the exhaust gas bypass valve is opened at a supercharging pressure of 310 mmHg, so that any further increase of supercharging pressure does not take place. As a result, the weight of the air supplied into the engine is reduced with a resultant decrease in the engine speed. As such, the engine performance is lowered at a high altitude than at a low altitude.